DETERMINING DEGRADATION EFFICIENCY IN BIOREMEDIATION PILOT TESTS AT THE GROUND-WATER/SURFACE-WATER INTERFACE IN A TIDAL WETLAND

The USGS began a study March 2002 to evaluate enhanced bioremediation methods to stimulate degradation of chlorinated ethanes, ethenes, and methanes near the ground-water/surface-water interface in the West Branch Canal Creek wetland area at Aberdeen Proving Ground, Maryland. Pilot tests of two technologies are being conducted—a bioaugmented, reactive mat constructed as a passive remediation barrier at the wetland surface, and direct injection of substrates and microorganisms at depth in the wetland sediments. An anaerobic dechlorinating microbial consortium (WBC-2) was derived from wetland sediment and developed for bioaugmentation applications. Innovative monitoring tools and microbial tests were used during the design phase and are currently being used during implementation of these pilot tests to determine the degradation efficiency of natural and bioaugmented sediment.

To assist in test design, effects of biostimulation and bioaugmentation were determined using four MICRO-Trac devices placed in wetland sediments in the pilot test area: an untreated control, a biostimulated treatment with a chitin-lactate mixture, a WBC-2 bioaugmented treatment, and a combined biostimulated and bioaugmented treatment. The combined biostimulated and bioaugmented treatment showed the highest methane production, highest removal of parent and intermediate contaminants, highest production of ethene, and highest population of dechlorinating microorganisms. Results confirmed the effectiveness of the selected substrates under field conditions and the need to bioaugment to achieve the high dechlorination rates required in a near-surface remediation effort. During monitoring of the reactive mat pilot test, modified wells and passive diffusion samplers are used to collect porewater from multiple depth intervals beneath and within the 22-inch-thick reactive mat. Concentrations of volatile organic compounds are used to calculate contaminant mass removal along upward flowpaths, and redox constituents and major ions are used to evaluate changing geochemical conditions generated in the mat. Bioactivity tests and molecular analyses are conducted periodically on small sediment samples from the mat to confirm the presence and degradation efficiency of WBC-2, providing evidence of removal of contaminants by biotic rather than abiotic processes.